1. Field of the Invention
The present invention relates to parts for and methods of repairing machines and is primarily but not exclusively concerned with high velocity fluid flow machinery such as turbines, and in particular turbine blades, nozzles, buckets and similar parts all hereinafter referred to for convenience as turbine blades.
Turbine blades in particular are subjected to wear during use and particularly in the case of steam turbines, the outer end of the turbine blades in a rotor assembly as the part which has the greatest linear speed in use and it is the outer leading end that is most prone to erosion.
Erosion generally occurs because of contact between the turbine blades and particles carried by the steam and also with the steam itself.
The high pressure end of a steam turbine, for example, is subjected to relatively hot steam and damage to the turbine blades is generally caused due to contact with particles of solid matter carried by the steam. At the low pressure end of the turbine the steam is at a low temperature and hence the water condenses more readily and considerable erosion occurs because of impact between the turbine blades and droplets of water.
2. Description of the Prior Art
Because of the known phenomenon of erosion, particularly at the leading outer edge of a turbine blade, it is quite usual to make that part of the blade as an insert made from a suitable hardened steel or from another hard material such as Stellite. Such an insert may be secured to the blade by brazing or welding for example.
Sometimes turbine blades are formed without such an insert and after same use when repair is necessary such an insert may be secured to the blades by cutting away the damaged part of the blade and securing, for example by brazing, a Stellite insert in position.
Whereas such inserts are very satisfactory and can prolong the life of the blade considerably, problems do occur in the securing of such inserts to the blade itself, such problems have at least partially been overcome in respect of certain types of blade by methods of repair proposed by the applicants and described in British patent specification Nos. 2, 091,139; 2,114,921; 2,124,126 and 2,144,361.
Problems occur during the securing of, for example, a Stellite shield to the blade because of the different nature of the materials to be joined together. A turbine blade is commonly made from a chrome steel, for example 12% chrome steel, and the securing by brazing or welding of the Stellite shield to such a blade involves the application of considerable thermal energy to the blade itself for which some compensation can be made by pre-heating the blade before the welding or brazing operation, controlled selective heating during the welding or brazing operation and post welding stress relief heating.
If such repairs can be carried out under controlled conditions then the chances of subsequent failure occurring are reduced. However, such careful control may require the complete dismantling of the turbine so that the blade to be repaired can be separated therefrom so as to be repaired in a gas chamber for example.
The cost of dismantling turbine blades is not only considerable from the labor cost involved but also the "down time" of the turbine is increased which can render such operations commercially unacceptable.
The cost of repairing a turbine blade which has not been provided with a Stellite shield and which is merely repaired by brazing or welding a new piece of chrome steel to the blade is relatively modest and a rather more simple operation.
Unfortunately, the long term wear characteristics of a Stellite shield are not provided and hence such a repaired blade will, in a relatively short space of time, required further repair.